1. Field of the Invention
The invention relates to a method of transmitting images from a transmitter to an individual receiver, comprising the steps of establishing a communication path between said receiver and the transmitter to obtain user selections identifying the images to be transmitted, assigning an image address to the images to be transmitted, and broadcasting each image along with said image address through a broadcast medium.
The invention also relates to a corresponding method of receiving such images from a transmitter.
2. Description of the Related Art
A known interactive television transmission system is disclosed in International Patent Application WO 96/38984, corresponding to U.S. Pat. No. 5,821,982. In this prior-art system, the images are teletext pages. A user establishes a connection with a service provider via a communication path (often a telephone connection). The user""s television receiver is tuned to a predetermined television station and set in the teletext mode. The service provider informs the user, by conventional speech, of a personal teletext page number and requests him to enter this number on his hand-held remote control unit. Subsequently, every time the user requests a new piece of information by means of the telephone keys, the requested page is updated and transmitted with the assigned personal page number. The receiver is arranged to continuously acquire the teletext page having said personal page number. Accordingly, whenever the page is updated and retransmitted, it will be captured and displayed by the receiver.
The teletext page for an individual user and similar pages for further individual users are transmitted along with other teletext pages constituting a regular teletext service. Other users may tune to the same transmitter and try to trace the personal page number of an interactively operating user. Since a teletext page number has a limited range (100-899) and the page numbers of the regular teletext service are known, it is not so difficult to trace such a personal page number. In order to reduce the risk that personal teletext pages appear on the display screen of xe2x80x9cco-viewersxe2x80x9d, the prior art patent application teaches the feature of extending the personal page number with a sub-code. By broadcasting a pseudo-page having the same page number but a different sub-code immediately after transmitting each personal page, the personal page appears on the display screen of a co-viewer""s receiver only for a short time.
As long as the use of interactive teletext is limited to playing a game or offering more general information, xe2x80x9cco-viewingxe2x80x9d is no practical drawback. However, an interactive teletext application offering personal information should be well protected against the risk of xe2x80x9cco-viewingxe2x80x9d.
The invention is not restricted to transmission of teletext pages. It is also possible to transmit video images at the standard television resolution. An example of such an interactive still image distribution system is disclosed in European Patent Application EP-A-0 396 062, corresponding to U.S. Pat. 5,014,125. A multiplicity of still video images requested by various users is transmitted as a standard television signal. Each image is encoded with an address identifying the user for whom it is intended. In this known system, the address is determined by the user terminal and communicated to the service provider through the telephone line. xe2x80x9cCo-viewingxe2x80x9d is not possible as long as all user terminals have different addresses.
It is an object of the invention to provide a method and arrangement for interactively transmitting and receiving images which further reduces the risk of xe2x80x9cco-viewingxe2x80x9d.
To achieve this object, the method in accordance with the invention comprises the step of communicating the assigned image address to tile receiver through the communication path prior to broadcasting the respective image. The corresponding receiving method is characterized by receiving the image address through said communication path, and applying said image address to the broadcast receiver prior to receiving each image.
The invention allows the image address to be dynamically modified by the transmitter. The modified address is communicated to the individual receiver only. In an interactive teletext system, this means that the sequence of personal teletext pages for an individual user is transmitted with different page numbers. The receiver automatically changes the requested teletext page number under the control of the remote service provider whenever a new page is about to be received. The addressed user does not notice this. He even does not need to enter a page number anymore, whereas a xe2x80x9cco-viewerxe2x80x9d can at most receive a single page of the sequence.
It is to be noted that remote control of a receiver by a service provider through the communication path is known per se from the afore-cited European Patent Application EP-A-0 396 062. The service provider can send a control message to the user terminal to instruct the television receiver to tune to an alternative channel if the predetermined channel lacks capacity. However, this control message does not modify the address assigned to the image and does not reduce the risk of xe2x80x9cco-viewingxe2x80x9d.
In a practical embodiment of the receiver, the communication means (usually an interface to the public switched telephone network) may be integrated in a television receiver. However, to allow conventional television receivers to be used for accessing an interactive (teletext) service, the communication means may be a separate interface box comprising an (infrared) output for applying the image address in the form of conventional remote control commands.